The goal of this project is to explain how ionizing radiation causes cancer, mutatation and cell death by elucidating the chemistry andenzymology of X-ray induced DNA damage and repair in human cells. The approach chosen uses a repeated sequence (alpha sequence) in human DNA. We will 1) determine the location of X-ray induced strand breaks within the alpha sequence; 2) determine the chemical nature of the 5' and 3' ends of X-ray induced strand breaks; 3) determine the effect of oxygenation, pH, temperature and free radical scavengers on the location and extent of strand breaks; 4) determine the location of strand breaks in intact cells; 5) determine rates and extent of repair of strand breaks after radiation; 6) develop a sensitive method for detecting modified bases in DNA radiated in vivo; 7) determine rates and extent of repair of base modifications after radiation; 8) develop substrates for isolation of human gramma-ray repair enzymes; and 9) determine whether these methods can explain the susceptibility of ataxia telangiectasia patients to X-ray damage and cancer. Knowledge of the processes of X-ray DNA damage and repair will be useful in developing more rational approaches to cancer prevention and therapy.